It is sometimes desirable to have an individual transistor device on a semiconductor integrated circuit in a conductive state in the absence of a supply voltage. Depletion-mode Field Effect Transistors (FETs) are commonly provided for this function. Depletion-mode Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) conduct current through a channel region in the absence of an applied voltage. However, under a bias condition, the normally “on” FET channel becomes depleted of charge carriers, effectively stopping channel conduction and turning the device off.
Such transistors are useful for applications such as eliminating charge build-up in sensitive circuitry. In one example application, a MOSFET may protect the read head of a mass storage disk drive from damage. The depletion mode device provides a low resistance path to ground when power to the head is turned off. During normal circuit operation, when the power is turned on, voltage is applied to the gate to remove the path to ground. Thus, the depletion-mode device suppresses static charge build-up while not interfering with circuit operation.
Although depletion-mode MOSFETs are effective devices for such applications, integration of these devices into an MOS fabrication process adds cost. It is a desire in the industry to provide methods and devices which implement depletion mode transistors at lower costs.